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Dynamic Range
When you take a picture of a house on a sunny day, you can set your camera to capture the bright features -- making it impossible to see details in the shady regions -- or to image the darker areas -- leaving the sunny areas overexposed. Every imaging system has a fixed difference between the brightest and dimmest features that can be detected. That difference is called the dynamic range. X-ray film has a limited dynamic range, meaning you can't use the same X-ray to look at dense and less-dense regions of the image. You'd need to take two or three exposures to see all the possible details. Digital radiography, which uses semiconductor detectors rather than film, has a much wider dynamic range that can be adjusted even after the exposure has been taken.
Image Contrast
Radiographic film collects X-rays that make it through the body. X-rays are blocked by dense, hard tissues -- such as bone -- and pass much more easily through the less dense tissues of the internal organs. There is a little bit of difference in the density of the liver and the kidneys, for example, but it is so slight that the contrast between the two types of tissue is very difficult to see. The situation is even worse when you're trying to distinguish healthy and diseased sections of the same organ. Conventional radiography is not the best choice for imaging soft tissue; ultrasound and Magnetic Resonance Imaging, or MRI, are better options.
Two-Dimensional Projection
Conventional radiographs are the result of X-rays propagating from one side of the body to the other at one location. If, for example, a chest X-ray image shows a dark blob on the right-hand side, you cannot tell if the denser mass is on the front of the ribcage, within the lung or on the patient's back. Also, because each image is the result of traveling through multiple layers of different tissues, it's difficult to see small differences in density in one of the layers. In computed tomography, this limitation is overcome by rotating the X-ray source and the detector around the body, creating a 3-D image of the interior tissues.
Ionizing Radiation
X-rays are in a class of electromagnetic radiation so powerful they can strip electrons right off of the atoms they hit. That ionization creates a chemical instability that can result in disruption of important biomolecules such as DNA. X-ray doses are limited because the damage they induce can trigger cancerous growths. Neither ultrasound nor MRI use ionizing radiation, and digital radiography requires lower doses of X-rays than conventional film radiography.
X-Ray Film
X-ray films are single physical entities. To get a diagnosis, the film must be exposed, developed and brought to the radiologist. When the diagnosis is complete, the images must be stored in a controlled environment so they do not degrade. In addition, if other consultations are necessary, the film -- or a copy -- must be physically distributed to other diagnosticians. Digital imaging methods create a computer file that can be processed almost instantly after exposure and distributed at will. Storage is also much less of a problem with digital picture archiving services.